This application claims Paris Convention priority of DE 101 18 885.4 filed Apr. 18, 2001.
The invention concerns a vacuum generator comprising an ejector nozzle which is connected to a compressed-air supply via a compressed-air line, and a first valve for opening and closing the compressed-air line.
Different kinds of vacuum generators are used to produce an underpressure. In the field of automation, vacuum generators are used which generate an underpressure using the Venturi principle. These vacuum generators are called ejectors and require compressed air for building up the underpressure. These vacuum generators are advantageous in that they are small and can rapidly produce an underpressure. Moreover, they usually do not have any moving parts.
For many applications, these ejectors are also provided as compact ejectors which have additional valves for switching the underpressure on or off in a simple fashion. These ejectors can also be provided with further elements, e.g. with vacuum sensors or vacuum switches to measure the underpressure level directly at the ejector nozzle and to subsequently pass on corresponding signals for controlling the valves in dependence on the measured values.
In this fashion, when a certain underpressure has been obtained, the control signals of the vacuum switch act directly on the valves and automatically control the valves in accordance with the desired values. The valves are e.g. switched off when a certain underpressure has been reached, and are switched on again when this underpressure falls below a preset value. Such a device is referred to as a regulated ejector. These ejectors have the substantial advantage that they consume compressed air only when an underpressure must actually be generated. The vacuum switches are usually electrical switches which, in turn, pass electrical signals.
These ejectors have the serious disadvantage that switching or control is no longer possible in case of power failure.
Prior art proposes construction of the electromagnetic valves of the ejector such that, in case of power failure, the compressed air is always applied at the ejector nozzle and a vacuum is always generated. This advantageously prevents the dropping of a vacuum-held load. However, energy is permanently consumed even when no underpressure is required.
To eliminate this disadvantage, ejectors have been developed with purely pneumatic control by constructing the vacuum switch as a pneumatic switch and replacing the electromagnetic valves with pneumatically controlled valves. This increases the control effort within the ejector and the pneumatic signals cannot be passed on to an electric control means (e.g. an SPS) without conversion. The pneumatic structural parts also have a shorter service life than electrically controlled structural parts.
In a further development, electrical and also pneumatic vacuum switches can be used. During normal operation, the electrical switch assumes the control and regulation function. The pneumatic vacuum switch is important only when the electrical switch is ineffective in case of power failure. Since the pneumatic vacuum switches are used in addition to the electrical vacuum switches, a switching cycle of the pneumatic switch is triggered simultaneously with each switching cycle of the electrical switch. The service life of such a system is therefore reduced to the service life of a purely pneumatic system. However, the service life of pneumatic vacuum switches is considerably less than that of electrical switches, since their construction includes a plurality of moving mechanical parts and diaphragms. Therefore, such vacuum generators are not susceptible to power failure but have a shortened service life.
For this reason it is the underlying purpose of the invention to provide a vacuum generator with high operational reliability as well as a long service life.
This object is achieved in accordance with the invention with a vacuum generator of the above-mentioned type by connecting a second electrical valve to the suction line of the ejector, which is open in the currentless state and which connects a pneumatic vacuum switch, which is connected in parallel to the first valve, to the suction line.
The inventive vacuum generator has a second electrical valve which is permanently electrically controlled to assume its closed position. In this closed position, the second electrical valve interrupts a connection between the suction line and the pneumatic vacuum switch to block switching thereof in response to the pressure in the suction line. The pneumatic vacuum switch assumes its rest position during driving of the second electrical valve.
In case of power failure, the second electrical valve can no longer be controlled and it assumes its rest position in which it is open. In this position, the second electrical valve connects the suction line to the pneumatic vacuum switch which is thereby loaded by the pressure in the suction line. Since the pneumatic vacuum switch is connected in parallel with the first valve, it takes on the function of the first valve which had assumed its closed rest position due to power failure.
The inventive vacuum generator can be controlled during normal operation via the electrical components. In case of power failure, the electrical components are ineffective and assume their rest position. The control function is then taken over by the pneumatic vacuum switch which is connected to the suction line.
The inventive vacuum generator has the substantial advantage that it retains its full function in case of power failure thereby correspondingly controlling the ejector nozzle. The service life of the vacuum generator is not impaired thereby since the pneumatic vacuum switch is not used during normal operation and assumes its function only in case of power failure.
In a further development, the operating point of the pneumatic vacuum switch can be set. The desired value of the underpressure is set through this operating point at which the vacuum switch changes from the closed into the open position or from the open into the closed position. Preferably, there are two operating points, an operating point for the maximum underpressure and an operating point for the minimum underpressure.
In a further development, the first valve and the second valve are connected via a piping connection to inhibiting members provided on the ejector nozzle. Both the first valve and the pneumatic vacuum switch can thereby control the ejector nozzle via this a piping connection.
Preferably, an electrical vacuum switch is provided for detecting the prevailing underpressure. This electrical vacuum switch determines the operating points of the first valve by controlling this valve at the desired maximum and at the desired minimum underpressure. This electrical vacuum switch cannot function during power failure and is replaced by the pneumatic vacuum switch.
Further advantages, features and details of the invention can be extracted from the following description which shows different switching situations of the inventive vacuum generator with reference to the drawing. The features shown in the drawing and mentioned in the claims and in the description may be essential to the invention either individually or collectively in any arbitrary combination.